1. Field of the Invention
The present invention relates to a chromium-free water reducible rust inhibitive paint for metals to protect the surfaces of ferrous metals, such as steel and pig iron, from rusting.
2. Description of the Related Art
As rust inhibitive paints for metals, those which contain, as a rust inhibitive pigment, a metallic zinc powder and a metallic aluminum powder, which exhibit the sacrificial corrosion inhibitive effect because of their greater ionization tendency than the main components of a substrate, such as iron, are well known, and a zinc rich paint is an example. Rust inhibitive paints for metals are classified into products which use an organic solvent, such as alcohol, and water reducible products which can be diluted with water. A chromic acid component (hexavalent chromium) is blended in a conventional water reducible binder solution of rust inhibitive paint for metals, for example, as in the water reducible rust inhibitive paint disclosed in U.S. Pat. No. 3,907,608.
This chromic acid component remarkably enhances the rust inhibitive performance by the sacrificial corrosion inhibitive effect of a rust inhibitive paint film by imparting the self-repair function to the rust inhibitive paint film and, at the same time, works as an inhibitor which forms a stable chemical film on particle surfaces of a metallic zinc powder and a metallic aluminum powder which are dispersed in an aqueous binder solution, thereby preventing reactions which occur between the aqueous binder solution and the metal powders. Therefore, a water reducible rust inhibitive paint for metals which contain chromic acid (hexavalent chromium) is excellent in rust inhibitive performance and has a long pot life.
In recent years, because the hexavalent chromium component has toxicity and carcinogenicity, a water reducible rust inhibitive paint for metals which contains a trivalent chromium component or a chromium-free water reducible rust inhibitive paint for metals which does not contain a trivalent chromium component capable of changing to hexavalent chromium has been desired, and several chromium-free water reducible rust inhibitive paints for metals have been proposed.
All of these water reducible rust inhibitive paints for metals which do not contain the chromic acid component are inferior in rust inhibitive performance to water reducible rust inhibitive paints for metals which contain the chromic acid component. Water reducible rust inhibitive paints for metals which do not contain the chromic acid component have such a problem that they are deficient in the self-repair function when a paint film is damaged, and when the aqueous binder solution is weakly acid or weakly alkaline, the metallic zinc powder and the metallic aluminum powder react with the water of the aqueous binder solution and generate hydrogen gas. When hydrogen gas is generated in a paint, traces of hydrogen bubbles remain in the formed paint film. The reaction increases viscosity to shorten the pot life of the rust inhibitive paint and impair the rust inhibitive performance.
In Japanese Laid-Open Patent JP 53-16044 A, there is disclosed, as a chromium-free water reducible rust inhibitive paint for metals, a rust inhibitive paint (a zinc rich paint) in which a metallic zinc powder is combined with an aqueous binder solution which is essentially composed of an organosilane-containing resin obtained by reacting organofunctional silane (including a water soluble silane coupling agent) with an organic resin, an organic silicate such as ethyl silicate, a high boiling point organic solvent such as etyl Cellosolve, and water.
Japanese Laid-Open Patent JP 6-41472 A discloses a corrosion inhibitive agent in which an almost neutral aqueous solution of the binder (pH=7) is prepared, using a water soluble synthetic resin essentially composed of a polymer containing α-β unsaturated fatty acid salts as a binder, and a rust inhibitive agent for metals is added as an inhibitor instead of chromic acid component.
U.S. Pat. No. 5,868,819 discloses an organic-solvent-based rust inhibitive paint which is essentially composed of a zinc powder, an epoxy resin, an epoxy-type silane coupling agent and a thermally expansible powder (a microcapsule powder in which a low boiling point hydrocarbon is enveloped in a solvent resistant shell wall).
Furthermore, there is a water reducible rust inhibitive paint for metals which is disclosed in Japanese Laid-Open Patent JP 10-46058 A. In this rust inhibitive paint for metals, a water soluble silane compound (a water soluble silane coupling agent) is used as a binder, and the paint contains a metallic zinc powder and a high boiling point organic liquid. Also, there is disclosed in the patent a metal product in which the painted surface is top coated with a silicious film in order to improve rust inhibitive performance.
Silicates (organic and inorganic) and colloidal silicas (water-based and solvent-based) are mentioned as the silicious substance, and alkyl silicates such as ethyl silicate are mentioned as the organic silicates. Japanese Laid-Open Patent JP 2002-121485 A discloses a water reducible rust inhibitive paint for metals in which an organic solvent having a boiling point of less than 100° C. is contained and which does not contain any organic resin. Also, there is disclosed a painted product having a top-coat of a silicious substance.
Japanese Laid-Open Patent JP 2001-64782 A discloses a rust inhibitive coating method which comprises forming an oxide film of Si, Al or Ti on a hot-dipped galvanized surface by the sol-gel process. In an example of the patent specification, a specimen is immersed in an alkoxide solution in a sol state (an alcoholic solution of a condensation polymerized alkoxysilane product which is obtained by mixing water and hydrochloric acid in an alcoholic solution of tetraethoxysilane), pulled up, dried and baked, thereby forming a silicious coating. However, in a salt spray test, white rusting occurred in a short time of not more than 12 hours and a rust inhibitive performance was not good.
Japanese Laid-Open Patent JP 6-9897 A discloses a technique which comprises immersing a metallic zinc flake in an aqueous colloidal silica solution, and forming a silica film on the surface of the metallic zinc flake thereby to prevent the metallic zinc flake from reacting with water. However, because the metallic zinc flake powder coated with an aqueous colloidal silica is apt to develop white rusting, the film is inadequate to prevent the reaction with an aqueous binder solution.
Japanese Laid-Open Patent JP 2003-3271 A discloses a corrosion inhibitive coating composition in which a metal powder surface-coated with a fatty acid, such as stearic acid, is used as a rust inhibitive pigment of a water reducible rust inhibitive paint for metals.
Conventionally, a flaky metallic zinc powder of a rust inhibitive pigment to be blended in a water reducible rust inhibitive paint for metals is protected by coating the surface of the metallic zinc powder with a fatty acid such as stearic acid or a higher alcohol such as lauryl alcohol in order to suppress reactions between the flaky metallic zinc powder and an aqueous binder solution. Specifically, a fatty acid or a higher alcohol is dissolved as a lubricant in an organic medium for flaking a metallic zinc powder to which a metallic aluminum powder can be mixed, the organic medium is removed by evaporating after this flaking processing, and stearic acid or lauryl alcohol is left on the surface of the flaky metallic zinc powder. In this coating method, however, stearic acid or lauryl alcohol works also as an adhesive to particles of the flaky metallic zinc powder and converts the flaky metallic zinc powder into secondary particles. Once secondary particles of a flaky metallic zinc powder are formed, it is difficult to disintegrate the secondary particles into primary particles again. If a paint is formed utilizing the flaky metallic zinc powder and goods are painted with the paint, the structure of secondary particles of the flaky metallic zinc powder remains in the coating, and it is difficult to form a thin coating which is excellent in rust inhibitive performance. It is possible to disintegrate secondary particles into primary particles by using, for example, a jet mill as means for disintegrating the secondary particles into primary particles. However, upon disintegrating into primary particles, the flaky metallic zinc particles are broken into fine pieces or the coating on the flaky particle surfaces is damaged. If a flaky metallic zinc powder which has been disintegrated into primary particles by such a method is dispersed in an aqueous binder solution to which a surfactant is added and a paint is formed, the flaky metallic zinc powder reacts with the aqueous binder solution to form hydrogen bubbles, the viscosity of the paint increases gradually and finally gelation occurs, thereby posing the problem that the pot life of the rust inhibitive paint is short.
Metal products which are painted with a conventional chromium-free rust inhibitive paint and metal products coated with the chromium-free rust inhibitive paint film and a top-coat on the painted film are inferior in rust inhibitive performance to metal products coated with a rust inhibitive paint film containing a chromium component. Therefore, a thin paint film suitable for the painting of fasteners such as bolts has a poor rust inhibitive performance, and white rusting and black rusting occur in a short time even if red rusting can be prevented to some extent.